Participants will be randomized to atomoxetine or matched placebo for 6 days. Study drug will be administered once daily at 40 mg/day on the first two study days, twice daily for days 3-5, & once on day 6. After discharge from the first component, and at least a 2-week washout period, participants will be re-admitted to the UCLA GCRC and switched to the opposite study medication for an additional 6 days.